a) Field of the Invention
This invention relates to an improved process for producing chlorine by oxidizing hydrogen chloride with an oxygen-containing gas in the presence of a catalyst at a low temperature and in a gas phase.
Chlorine is produced on a large scale by the electrolysis of sodium chloride. In spite of the ever-increasing demand for chlorine, the demand for caustic soda which occurs simultaneously upon the electrolysis of sodium chloride is lower than that for chlorine. Difficulties have thus been encountered in meeting their different demands appropriately.
On the other hand, a great deal of hydrogen chloride is obtained as the by-product of the chlorination or phosgenation reaction of an organic compound. Since the volume of this hydrogen chloride is significantly higher than the demand level for hydrochloric acid, lots of hydrogen chloride is disposed of wastefully without its utilization. Furthermore, the treatment cost for its disposal has reached a considerable value.
It is thus possible to meet the demand for chlorine without developing an imbalance between its production level and that of caustic soda, provided that chlorine can be efficiently recovered from hydrogen chloride which is disposed of in a large volume as described above.
b) Description of the Prior Art
The reaction in which hydrogen chloride is oxidized to produce chlorine has been known as the Deacon reaction for many years. The copper-based catalysts which Deacon invented in 1868 have conventionally been considered to show the best activities. A number of proposals have been made on catalysts which contain various compounds added as a third component to copper chloride and potassium chloride. In order to oxidize hydrogen chloride at an industrially-satisfactory reaction velocity with these catalysts, it is however necessary to raise the reaction temperature to 450.degree. C. or higher. At such high reaction temperatures, a problem arises that their catalytic service life is shortened due to scattering of their catalyst components. In addition, the oxidation reaction of hydrogen chloride has an equilibrium point. The yield of chlorine decreases as the temperature goes up. It is thus required to develop a catalyst which possesses activity at a which is as low as possible.
From the above-described viewpoint, iron-base catalysts and some other catalysts have been proposed as catalysts other than copper-based catalysts. However, the present inventors are still not aware of any catalysts which may exhibit fully practical quality and performance. It has also been proposed to employ chromium oxide as a catalyst for the oxidation of hydrogen chloride because chromium oxide has better stability and durability at elevated temperatures than copper and the like. As far as the present inventors are aware of, none of such chromium oxide catalysts have yet been reported to have shown satisfactory activities. For example, it is disclosed in U.K. Patent Specification No. 584,790 that hydrogen chloride is caused to flow at about 400.degree. C. over a catalyst, which has been prepared by impregnating a suitable carrier with chromic anhydride or an aqueous solution of chromium nitrate and then subjecting the thus-impregnated carrier to pyrolysis, to produce chlorine; and after deactivation of the catalyst, the supply of hydrogen chloride is cut off and air is in turn caused to flow so as to regenerate the catalyst, and the supply of air is then stopped and the supply of hydrogen chloride is instead resumed. In U.K. Patent Specification No. 676,667, hydrogen chloride and an oxygen-containing gas were reacted at 420.degree.-430.degree. C. in the presence of a catalyst composed of a carrier and a bichromate or heavily dark green chromium oxide supported on the carrier, whereby hydrogen chloride conversions of 67.4% and 63%, both based on the equilibrium value, were achieved at hourly space velocities of 380 hr.sup.-1 and 680 hr.sup.-1 respectively. The occurrence of the reaction was still observed at 340.degree. C. However, the hourly space velocity was controlled at such a low level as 65 hr.sup.-1 and the resultant conversion was as low as 52%.
As mentioned above, even when chromium oxide is used as a catalyst, it is not of such quality that it can withstand an industrial operation because the reaction temperature is high and the hourly space velocity is low in each of such conventional processes. In other words, the chromium oxide catalysts which have conventionally been reported are not believed to exhibit any specifically excellent performance compared with the copper-base catalysts.